The speed of sound is a term used to describe the speed of sound waves passing through an elastic medium.
The speed varies with the medium employed (for example, sound waves move faster through water than through air), as well as with the properties of the medium, especially temperature.
The term is commonly used to refer specifically to the speed of sound in air.
At sea level, at a temperature of 21 degrees Celsius (70 degrees Fahrenheit) and under normal atmospheric conditions, the speed of sound is 344 m/s (1238 km/h or 770 mph).
The speed varies depending on atmospheric conditions; the most important factor is the temperature.
Humidity has little effect on the speed of sound, nor does air pressure by itself.
Air pressure has no effect at all in an ideal gas approximation.
This is because pressure and density both contribute to sound velocity equally, and in an ideal gas the two effects cancel out, leaving only the effect of temperature.
Sound usually travels more slowly with greater altitude, due to reduced temperature.